1. Field of the Invention
The present invention relates to an operational amplifier and, particularly, to an operational amplifier that includes a capacitor between an input node and an output node of an output stage amplifier.
2. Description of Related Art
In semiconductor devices, operational amplifiers are widely used to amplify signals to be handled. An example of operational amplifiers is disclosed in Japanese Unexamined Application Patent Publication No. H6-326529 (U.S. Pat. No. 5,311,145). An operational amplifier includes an input stage amplifier and an output stage amplifier. The input stage amplifier converts an input signal according to the input structure of the output stage amplifier and amplifies the input signal. Further, the input stage amplifier sets idling currents flowing through transistors constituting the output stage amplifier. The output stage amplifier amplifies the signal generated in the input stage amplifier and outputs the signal.
In the operational amplifier, defects such as oscillation possibly occur if the phase margin of the operational amplifier is small. In order to increase the phase margin of the operational amplifier, a phase compensation capacitor may be placed between an input node and an output node of the output stage amplifier. FIG. 16 shows a block diagram of an operational amplifier 100 that includes a phase compensation capacitor.
Referring to FIG. 16, the operational amplifier 100 includes an input stage amplifier 110 and an output stage amplifier 111. The inverting input terminal of the input stage amplifier 110 is connected to an output terminal Vout, and the non-inverting input terminal of the input stage amplifier 110 is connected to an input terminal Vin+. The output of the input stage amplifier 110 is output as a single-end signal. The input stage amplifier 110 outputs a current I. The output stage amplifier 111 inverts the single-end signal that is output from the input stage amplifier 110 and outputs the inverted signal. Further, a capacitor C to serve as a phase compensation capacitor is connected between an input node and an output node of the output stage amplifier 111.
Thus, the operational amplifier 100 functions as a buffer in which the output terminal is connected to the inverting input terminal of the input stage amplifier 110. The slew rate SR of the operational amplifier 100 is represented by the following expression (1):
                    SR        =                                            ⅆ              Vo                                      ⅆ              t                                =                      I            C                                              (        1        )            
In the above expression (1), Vo indicates the voltage of the output terminal, t indicates time, I indicates the output current of the input stage amplifier 110, and C indicates the capacitance of the capacitor. The expression (1) shows that the slew rate decreases when the capacitance of the capacitor increases and the slew rate increases when the current output from the input stage amplifier 110 increases.